1. Field of the Invention
The present invention relates to an image formation method capable of producing a full-color image by superimposing four yellow, magenta, cyan and black toner images in the field of electrophotographic recording, a set of color toners for use in the above-mentioned full-color electrophotographic image formation method, and a color image formed by the above-mentioned full-color image formation method.
2. Discussion of Background
A color copying machine and a color printer employing the electrophotographic process have been widely used in recent years. In the electrophoto-graphic process for producing a color image, there are employed yellow, magenta and cyan toners, optionally a black toner being added thereto when necessary.
The color image is required to have a proper degree of glossiness. To fulfill the above requirement, a binder resin with a relatively low softening point is conventionally used for a color toner so as to make the surface of a toner layer constituting a color image area flat.
The amount of color toner deposited on a transfer sheet by development widely varies in a color image such as a full-color photographic image formed by the electrophotographic method of producing a full-color image. To be more specific, chromatic color images include many portions with the deposition amount of a color toner being small, while a black image is provided with a large amount of a black toner. The conventional full-color image in which the deposition amount of color toner shows some scatter, as mentioned above, has the shortcoming that the glossiness tends to be uneven in the entire image. In addition, the glossiness of the black image portion is extremely high, and the image will become dull as a whole.
The glossiness of the black image portion can be reduced when the black toner comprises as the binder resin the same resin as employed in the monochrome toner or a polymeric resin with a relatively high softening point. In such a case, however, there is the problem that a photographic image including a black image portion, such as a photographic image of a man's face, becomes considerably unattractive. In particular, when a color image includes a character image and a photographic image, both the character image and the photographic image become dull because there is a great difference in glossiness between a highlight portion of the photographic image and the character image portion.
There is a proposal in Japanese Laid-Open Patent Application 8-220821 to solve the problem of uneven glossiness in the full-color image. Namely, full-color images are formed by use of chromatic color toners, that is, yellow, magenta and cyan toners in combination with a transparent toner capable of producing a transparent image with a specified glossiness. This method, however, cannot be applied to the conventional electrophotographic copying machine because the transparent toner is additionally employed.